I am afraid I am hopelessly inexperienced when it comes to these issues, but I would think it both inappropriate and awkward for you to tell her, especially since you are in the early stages of the relationship.
Do you know if the problems are general problems? Does she always dress like that? Or is it just when she’s dating and hoping to impress? It makes a big difference.
If you can get a mentor of hers to tell her (rabbi, rebitzen, <probably not parents>) not ??? you, it would be the best.